Re: [dpdk-dev] [PATCH v6 2/3] gro: add VXLAN UDP/IPv4 GRO support

["Hu, Jiayu" <jiayu.hu@intel.com>]

Fragments of a flow are sorted by frag_oft, but they may have different
timestamp. For example, there are three fragments, whose frag_oft is:
frag[0].frag_oft=0, frag[1].frag_oft=4, frag[2].frag_oft=6; and they are
fragments of one UDP packet but are not neighbors. In the first RX burst,
host receives frag[1] and calls rte_gro_reassemble(), and we assume the
timestamp of frag[1] is 10; in the second RX burst, host receives frag[0]
and also call rte_gro_reassemble(), and timestamp of frag[0] is 11; the
third time, host receives frag[2] and timestamp of frag[2] is 12. The three
fragments are stored in three items of a UDP GRO table:
items[0]: frag[0], timestamp is 11
items[1]: frag[1], timestamp is 10
items[2]: frag[2], timestamp is 12
Now we want to flush packets whose timestamp is less than or equal to
10. frag[1] should be returned, but in your code, no packets will be flushed.
Because the timestamp of items[0] is greater than 10, the left two fragments
will not be checked. This is what I want to say.

From: yang_y_yi <yang_y_yi@163.com>
Sent: Tuesday, September 22, 2020 9:44 AM
To: Hu, Jiayu <jiayu.hu@intel.com>
Cc: dev@dpdk.org; thomas@monjalon.net; yangyi01@inspur.com
Subject: Re:Re: [dpdk-dev] [PATCH v6 2/3] gro: add VXLAN UDP/IPv4 GRO support
Importance: High

BTW, start_time is checked for the first packet in a flow, gro_udp4_merge_items(tbl, j) will merge all the packets in this flow once if they can be reassembled, gro_udp4_merge_items(tbl, j) doesn't check start_time, so this still can let some new items in this flow have chance to be merged.

At 2020-09-22 09:29:38, "yang_y_yi" <yang_y_yi@163.com<mailto:yang_y_yi@163.com>> wrote:

>Thanks Jiayu, I have fixed other comments except this one:

>

>

>

>>The items of a flow are ordered by frag_oft, and start_time

>>of these items is not always in ascending order. Therefore,

>>you cannot skip checking the items after the item whose

>>start_time is greater than flush_timestamp. This issue also

>>exists in UDP/IPv4 GRO, and need to correct them both.

>

>

>I think the issue here is if we should strictly follow flush_timestamp, it is possible there are new items in items chain. we have chance to merge more packets if we don't follow flush_timestamp. So an ideal change can be this. But is it acceptible if we don't use flush_timestamp? It can flush some packets in advance therefore miss next merge window. Maybe current way is most resonable and a tradeoff between two exterem cases.

>

>

>

>

>

>diff --git a/lib/librte_gro/gro_udp4.c b/lib/librte_gro/gro_udp4.c

>index 061e7b0..ffa35a2 100644

>--- a/lib/librte_gro/gro_udp4.c

>+++ b/lib/librte_gro/gro_udp4.c

>@@ -391,7 +391,6 @@

>

>                j = tbl->flows[i].start_index;

>                while (j != INVALID_ARRAY_INDEX) {

>-                       if (tbl->items[j].start_time <= flush_timestamp) {

>                                gro_udp4_merge_items(tbl, j);

>                                out[k++] = tbl->items[j].firstseg;

>                                if (tbl->items[j].nb_merged > 1)

>@@ -407,12 +406,6 @@

>

>                                if (unlikely(k == nb_out))

>                                        return k;

>-                       } else

>-                               /*

>-                                * The left packets in this flow won't be

>-                                * timeout. Go to check other flows.

>-                                */

>-                               break;

>                }

>        }

>        return k;

>